1. Field of the Invention
The present invention relates to an apparatus and a method for processing an image signal and more, particularly, to an image signal processor and an image processing method in which data of an image is compressed at a fixed coding rate, and which have, as a pre-processing before compression processing, a step of converting the image so that the amount of information corresponding to high-frequency components of the values of pixels in the image is reduced.
2. Description of the Related art
When a digital image is recorded on a predetermined recording medium, the amount of data of the image to be recorded can be reduced by using compression processing represented by processing in accordance with the Moving Picture Experts Group (MPEG). By such compression processing, the amount of data after processing can be varied. However, if the amount of data after compression is excessively small, digital noises such as block noise and mosquito noise become perceptible, resulting in a deterioration in image quality after expansion. To prevent such a deterioration in image quality after expansion processing, therefore, a certain amount of data is required after compression.
Conventionally, image data is processed in a tentative way (first-path processing) before it is actually compressed, and a coding rate is determined according to the amount of data by considering the capacity of a transmission line and a recording medium and so on. Thereafter, the data is actually compressed (second-path processing). In this manner (by variable rate coding), a deterioration in image quality after compression and expansion of the image can be prevented.
In the conventional art, if an image is encoded by only one path of compression processing and if the encoded data is transmitted through a transmission line and recorded on a recording medium, it is necessary to change (increase) the coding rate and to sufficiently increase the capacity of the transmission path and the recorded medium (in the case of apparatuses for business use or for reproduction only).
It is possible that coding is performed at a low coding rate which is not allowed to be changed, as in the case of an apparatus operated both for recording and for reproduction by a user. In such a case, coding is possible if the amount of data is always small. However, a deterioration in image quality after expansion results under such a condition. Thus, in the conventional art, it is difficult to achieve real time coding (by only one path of compression processing) without causing a deterioration in image quality after expansion.